Need help about SPAM

Hi,
I have a Spearmail for distibuting the emails in my intranet. It’s a windows based software and there is no any security to block the unknown / spam mails, I am facing the daily problem of spam lots of trojans, malwares are downloaded to my mail server machine, also my domain is blacklisted in RBL
So please suggest me what is a best way to resolve this issue.